Fuel cell power plants are well known and may be used, for example, as power sources for electrical apparatus in space vehicles, power sources in automotive applications, and as stationary electricity generators for buildings. In a typical fuel cell power plant, multiple fuel cells are arranged together in a repeating fashion to form a cell stack assembly (“CSA”). Each individual fuel cell in the CSA typically includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant fuel (e.g. hydrogen) is supplied to the anode, and a reactant oxidant (e.g. oxygen or air) is supplied to the cathode. The hydrogen electrochemically reacts at a catalyzed area of the membrane or anode to produce hydrogen ions and electrons. The electrons are conducted to the cathode through an external load. The hydrogen ions transfer through the electrolyte to the cathode, where they combine with the oxidant and the electrons to produce water and thermal energy.
In a fuel cell with a proton exchange membrane (“PEM”) as the electrolyte, the combination of the electrolyte and the anode and cathode is oftentimes called a membrane electrode assembly (“MEA”). In PEM fuel cells, the membrane must be hydrated in order to obtain optimum membrane performance and membrane life. PEM fuel cells typically operate at about 80 degrees C., and if dry reactant gases are used, the water in the membrane, and that produced by the fuel cell, can evaporate into the gas stream and leave the membrane dehydrated. In order to prevent membrane dehydration, the reactant gasses are frequently humidified using external and or internal humidification means.
External humidifiers humidify the reactant gases prior to supplying the gas to the fuel cells. Internal humidifiers supply water, such as coolant water, directly into the reactant gas passages inside the fuel cells. The reactant and coolant passages are separated by, for example, a porous plate or water permeable membrane. A pressure differential between the coolant and reactant can force part of the coolant water through the plate or membrane and into the reactant passages to humidify the reactant. An advantage to using an internal humidifier is that the fuel cell power plant can be made simpler and more compact than one using external humidifiers. An example of a PEM fuel cell with internal humidification is shown in U.S. Pat. No. 4,826,741, issued to Adlhart et al. on May 2, 1989.
A drawback to internal humidification stems from the difficulty in providing a uniform distribution of water in the CSA. For example, inadequate control of the pressure differential between the coolant and reactant can result in a mal-distribution of water, which degrades cell performance. If the coolant pressure is too low humidification can be insufficient, which increases internal ohmic resistance. If the coolant pressure is too high, excess water can enter the gas passages and flood the electrodes.
Another difficulty with internal humidification is controlling the water distribution in the plane of the active area of the MEA. The active area of the MEA is the portion of the membrane (or an adjacent electrode) that is catalyzed, and it is where the electrochemical reaction takes places. If the coolant pressure is sufficient to humidify the upstream region of the active area, water may be excessive in the downstream region, due to the progressive consumption of the reactants and the production of water by the electrochemical reaction. This can result in flooding of the catalyst in the downstream regions. Attempts to decrease the liquid water in the downstream region can involve reducing the humidity of the incoming reactant, but this can lead to membrane drying in the upstream region.
Another difficulty with internal humidifiers is associated with need for transient capability. When a fuel cell is operating, the flow rates (and pressures) of reactant gases will change according to changing or transient load demands placed upon the fuel cell. If the coolant pressure cannot also change, or change as quickly as the reactant pressures, then transient load demands may cause the pressure difference between the coolant and the reactants to increase or decrease to levels where the electrodes flood or the membrane becomes dehydrated.